Ride-hailing service Uber wants to graduate from being an app to being a button in any app, and its latest partner also helps accentuate Uber's role as an alternative to drinking and driving.

A new partnership between Uber and smartphone breathalyzer startup Breathometer lets users who might have had a few too many call an Uber directly from the Breathometer app. That makes the breathalyzer company the latest in a string of selected partners who are allowed to integrate directly with Uber's app. An easy Uber button drives ride-hailing business, and it can often streamline a customer experience. Just measured your BAC? Call an Uber home. Looking to buy a house? Call an Uber to go tour it. Just made a restaurant reservation or bought a concert ticket? Take an Uber there.

But this particular partnership with Breathometer benefits Uber in another way. The company has sought to brand itself as a force against drunken driving through partnerships with the NFL and Mothers Against Drunk Driving. It even trotted out a report this month that said Uber likely reduced drunken driving crashes.

The one problem? The report's stats are not as conclusive as they seem. The study said that alcohol-related crashes for those under 30 dropped more in cities where Uber operates than cities where it doesn't, which suggests Uber "likely prevented" 1,800 crashes over the past two and a half years. But that's just a correlation -- there's no way to show that Uber caused that drop. Even MADD, the study's partner, hesitated to draw a direct cause-and-effect relationship when asked about the study by ProPublica. "Nobody is saying that there is a causation relationship here," MADD's senior vice president of marketing and communication Amy George told ProPublica. "This is a correlation relationship. Purely correlational." (Update: MADD clarified that they "absolutely stand by the report's findings" and are not backing away from the report. "Uber is a powerful tool to reduce drunk driving," MADD said. "We have consistently supported the strong correlations outlined in the report, which show that Uber is having an impact.")

Uber is certainly a helpful new option for people who might otherwise drive drunk. But if the company can't prove that its service reduces alcohol-related accidents, it can still align itself with apps that try to prevent drunken driving.

Uber was determined to nail down a partnership with Breathometer, said CEO Charles Michael Yim.

"We were actually in heavy discussion with Lyft -- we played around with their API as well," Yim said. "Then Uber got really serious with this. We made a conscious decision to back away from Lyft." Breathometer's partnership with Uber is exclusive, meaning Lyft and other ride-hailing services can't also partner with them.

Breathometer has been piloting its integration with Uber since October and has sent 1,700 users to the Uber app in the past nine weeks, the company said. Any user who breathes a 0.04 BAC or higher will be prompted to "Get Home Safe" -- a screen that used to list local cab companies. Now, it shows a button to call an Uber, along with an estimate of how far away the nearest one is.

Breathometer makes two smartphone alcohol breathalyzers -- the $50 original, which connects through a headphone jack, and the $100 Breeze, which works over Bluetooth and can fit on a keychain. The company is also working on a similar device that will measure breath quality and hydration levels.